Aggregates
A: In addition to the standards of
sand, peat,
and Sphagnum, some people include
perlite, pumice, and gravel in their planting mixes. These are called aggregates. They all serve the same purpose as sand
of providing drainage, but have slightly different attributes.

Perlite is a light, crumbly, white rocky material. It is very nice if you
want to provide good drainage to a mix--it does not retain water.
A pot full of perlite is very light! It is white, so reflects sun well---a useful consideration
if you have pots in full sun and they are overheating.

In contrast, pumice is denser and
is a nice alternative to perlite if you want some ballast in the pot. This
is important if you are growing a tall plant like Heliamphora which
might
topple a pot filled with a light perlite mix. Lava is similar to pumice, and is often red or black.

Gravel is denser still, and makes a very heavy medium if it constitutes a significant fraction of the soil mix.

You should wash any of these products before using them to remove the fines (tiny particulates).
See my instructions on washing sand. Do not
breathe the dust from these products as they could cause lung ailments.